Arrangement for variable adjustment of band width



April 15, 1941. WINKLER 2,238,763

ARRANGEMENT FOR VARIABLE ADJUSTMENT OF BAND WIDTH Filed Nov. 20, 1937 [L] g 'z I Q S f -FREQUENCY INVENTOR. Jaws NKLEI2.. BY

Patented Apr. 15, 1941 ARRANGEMENT FOR VARIABLE ADJ US T- DIENT OF BAND WIDTH Johannes Winkler, Berlin, Germany, assignor to Telefunken Gesellschaft fiir -Drahtlose Telegraphic in. b. 11., Berlin, Germany, a corporation of Germany Application November 20, 1937, Serial No. 175,593 In Germany December 21, 1936 5 Claims.

It is known that a bandpass filter of two resonant circuits coupled together can be made variable by changing the inductive coupling. When a change of the coupling is attempted in practice through the moving of one of the coils a serious difliculty isencountered as soon as the bandpass filter coils are placed under a shield. The tilting of one coil about its axis with respect to that of the other changes the position of the tilted coil with respect to also the shield and the tuning becomes disturbed. This same holds true when one of the coils is moved up or down alon their common axis. In this latter case the distance between the movable coil and a part of the shield is changed.

An attempt has been made to overcome this difficulty by the construction of two telescoping can-shaped shielding members in which the coils were placed. For a change in the inductive coupling the coils are moved together with their respective shields along their common axis. This arrangement is naturally very clumsy and in the case of iron-cored coils it has the added disadvantage that the tuning becomes disturbed as soon as the distance between the iron cores is changed.

Pursuant to the invention another known method of changing the coupling is employed in a hereinafter described manner. This method of changing the coupling is realized by moving back and forth along the common axis of the main coils of the two resonant circuits, a smaller coil placed between the two coils and connected in series with the main coilof one of the resonant circuits. The innovation lies in the fact, that by this method of band width adjustment the main coil and the small coupling coil of one resonant circuit and the coil of the other resonant circuit, mounted side by side along a common axis can be placed under a single shield and that the small coupling coil placed between the two other coils can be moved back and forth along the common coil axis.

In the drawing Figure 1 illustrates one example of constructional arrangement pursuant to the invention, Figure 2 is a diagrammatical representation or circuit diagram of the embodiment disclosed in Figure 1, and in Figure 3 are shown selectivity or resonance curves obtained with the embodiment disclosed in Figures 1 and 2. In Figures 1 and 2 the coils are designated by the same reference numbers. In Figure 1, under the shield can A is the tube R made of insulating material which serves for the permanent mounting of coils L1 and 1c. The members shown in dotted line are the iron cores, the position of which is adjustable through openings in the shield (not shown) for tuning in a manner well known. These cores are ferromagnetic and consist of finely divided individually insulated magnetic A further essential advantage of the inventionresides in the fact that the sliding movement of a small coil requires less constructional expense than if the main coils were made movable, especially when they have iron cores.

In Figure 3 there are shown the selectivity curves for the narrow and wide band adjustments of the circuit. Adjustment of the small coil L2 towards the fixed coil L3 will produce close coupling between the two coupled circuits and the selectivity curve for that condition will be the substantially flat topped curve of Figure 3, whereas adjustment of the small movable coil L2 in the direction away from the coil L3 will produce a loose coupling between the coupled circuits and the selectivity curve for that condition of adjustment will be substantially sharp and narrow. In a trial experiment, for a bandpass filter tuned to an intermediate frequency of 490 kc./s. the coil L1 had 180 windings, L2 15 windings and L3 170 windings. The winding ratio of L2 to L1 therefore is 1:12 giving an inductance ratio of 1:144 in the case of aircores. Since the example illustrated herein relates to iron cores, the ratio is greater than 1:300.

This bandpass filter is primarily intended for the intermediate frequency portion of a receiver, although it is fundamentally applicable to the received frequencies as well. The variable coupling can also be combined in a known manner, for instance, for simultaneous control adjustment with the tuning or else it can be made adjustable by hand.

What I claim is:

1. The combination with a pair of tuned circuits having main reactances coaxially disposed in fixed spaced relation, of means for varying the coefiicient of coupling between said reactances, said means comprising an auxiliary inductance coaxially arranged between the main reactances, said auxiliary inductance being disposed closer to one of the reactances than to the other and being serially connected to said other reactance, a shielding can enclosing the reactances and the auxiliary inductance, and means for adjusting the position of the auxiliary inductance from outside the shielding can.

2. The combination with a pair of tuned circuits having main inductances which are coaxially disposed in fixed spaced relation, of means for varying the coeificient of coupling between said inductances, said means comprising an axially adjustable auxiliary inductance connected in circuit with one of said main inductances and in inductively coupled relation with the other of said inductances, a shielding can enclosing all of said inductances, means arranged externally of the shielding can for adjusting the axial position of said auxiliary inductance and thereby to vary said inductive coupling.

3. The combination with a pair of tuned circuits having a pair of main inductances coaxially disposed on a common support and in coupled relation, of a shield can enclosing said inductances, said shield having a disturbing effect on the tuning of said circuits with variation of the coupling by coaxial movement of the inductances, means for avoiding said disturbing efl'ect with change of coupling comprising a small auxiliary coil adjustably mounted on said support and disposed between the main inductances which pled tunable circuits, the coils of which are coaxially mounted in fixed relation within a common shielding can, the coil of one of said circuits having in series with it a smaller coil which is also coaxially mounted and closer to the coil of the other circuit, said smaller coil being adapted for axial adjustment to vary the band width without disturbing the tuning, and means arranged externally of the shielding can for making said adjustment.

5. A bandpass filter according to claim 4, wherein adjustable iron cores are associated with the fixed coils to vary the tuning of the circuits.

J OHANNES WINKLER. 

